Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe, and is purified therein. After this step, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the outside air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide and hydrocarbons (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through an oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DeNOx catalyst. The LNT catalyst absorbs the NOx contained in the exhaust gas when air/fuel ratio is lean, and releases the absorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich.
If a temperature of the exhaust gas, however, is high (e.g., the temperature of the exhaust gas is higher than 400° C.), the LNT cannot purify the nitrogen oxide contained in the exhaust gas. Particularly, if a vehicle is accelerated quickly or runs at a high load condition, the temperature of the exhaust gas is high and the LNT disposed closely to the engine cannot purify the nitrogen oxide in the exhaust gas. In addition, since a flow of the exhaust gas is large at the quick acceleration condition or the high load condition, NOx exhausted to the exterior of the vehicle may be increased. Therefore, a purifying of the nitrogen oxide may be deteriorated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.